Drainage elements or slabs of this kind are often not only draining, but also heat-insulating and/or capillary-breaking. They are usually produced by the molding compound being pressurized and dispersed through a nozzle orifice down into passing open face molds, which move in a circulatory path. Once a mold is filled, an accompanying cover is pressed down into the mold to compress the molding compound, so that the spherical granulates adhere to one another to such an extent that the element attains the desired strength. During the compression, the mold and the cover are flowed through by warm air to harden the bonding agent, after which the cover is removed and the drainage element is forced out of the mold with the aid of ejectors in the mold.
One problem with this method is that the expanded pellets of the granulate do not get evenly distributed when the compound is dispersed in the mold, so that in some areas they become less tightly packed, resulting in the formation of cavities in the molding compound. When the elements are then compressed and hardened, the cavities can remain and impair the strength of the element, especially the compressive strength, as is indicated in the appended fig.
In addition, the molds have defined depths, so that only elements of a given thickness can be produced with one and the same device.